A child is a human being past the point of birth and under the age of majority.
As your definition doesn’t actually technically include the en-wombed either, perhaps you also need a new term for “pre-born humans”… you know, except that we already have one — “fetus”. This is a great term, and covers pretty much everything from zygote to just-before-birth — so versatile.
Murder is unlawful killing of a human being.
So killing is an acceptable term. You get to kill things to survive, it’s how the world works, otherwise no one eats. (Remember, if it tries to eat you, kill it.)
And again — until medical science catches up with the stick-it-to-them-morality, the argument that a fetus needs to stay in someone else’s body is merely an argument of your desire vs their bodily autonomy. They should automatically win. Perhaps in situations where the fetus can be removed without trauma to either host-mother or fetus and survive outside you’ve got some kind of argument, but when it can’t? You’re just being petty and cruel all around. That’s some kind of weird anti-living-person pro-life thing you’ve all got going on.
You get to grow whatever you want in your very own body. Have at, just give other people the same courtesy.